1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device, and more particularly to a metal electrode of a SAW device, for example, an inter-digital transducer (IDT) electrode made of a comb shaped aluminum film formed on the surface of a piezoelectric substrate.
2. Description of the Related Art
A conventional SAW device having an IDT electrode is formed on a piezoelectric substrate (e.g., a LiTaO3 substrate). FIG. 13 is a cross-sectional view of the IDT electrode structure of the conventional SAW device. As shown in FIG. 13, a conductive film 2 (an aluminum film 2) is formed on the piezoelectric substrate 1. A photolithography etching of the conductive film 2 is performed to form the IDT electrode pattern.
FIG. 14 is a cross-sectional view of another IDT electrode structure of the conventional SAW device. As shown in FIG. 14, the IDT electrode is formed as a laminate structure, and includes a conductive film 3, a second conductive film 4, and a third conductive film 5, formed on the piezoelectric substrate 1.
Recently, as mobile communication systems have become to operate at higher frequencies, the operating frequency of SAW devices has increased from hundreds of MHz to several GHz, and the power output demand has increased as well. As a result of the increase in frequency, the pattern width of the IDT electrode must be made finer.
When a large electric power is applied to a SAW device having a fine electrode width, a wrap generated by a surface acoustic wave causes stress on the electrode film. When such a stress exceeds stress limit of the electrode film, aluminum atoms in the electrode film material move in the grain boundary. As a result, hillocks in the electrode and voids of a surface of the electrode are formed, and the characteristics of the SAW device deteriorate. Further, the electrode of the SAW device may be broken due to a short of the electrode.
To prevent hillocks and voids, the conventional SAW device is disclosed in, for example, Japanese Laid-Open Patent Publication: HEI09-199979, published on Jul. 31, 1997. In the conventional SAW device, the aluminum material is used as a main component of the electrode. Also, attempts have been made to reinforce the electrode film by adding, for example, copper (Cu), titanium (Ti), nickel (Ni), magnesium (Mg), palladium (Pd), and so on. Specifically, since copper (Cu) has a low resistance and can withstand a higher stress migration and a higher electrical migration, a copper alloy film including an inorganic protecting film is proposed as the electrode film of the SAW device. At this time, the inorganic protecting film can inhibit an oxidation of the copper, and the inorganic protecting film is, for example, a silicon oxide (SiO2) having a thickness of 2 nm through 30 nm.
However, the propagation properties of the surface acoustic wave are deteriorated due to the inorganic protecting film. Further, it is too difficult to etch the copper effectively.